The present invention relates to a cathode structure for a magnetron, and more particularly to an improvement in the structure of an upper end shield for supporting the upper end of a filament and in the structure of a getter fixedly arranged on the upper end shield.
As typical self-oscillation type microwave electron tubes, magnetrons have been widely used. As is well known, a magnetron comprises an anode of a divided cavity structure and a cathode surrounded by the anode. With this magnetron, based on the interaction between electrons emitted from the cathode and a high frequency electric field, high frequency power is produced as a microwave oscillation output.
FIG. 1 is a longitudinal sectional view schematically illustrating an example of a conventional cathode structure for a magnetron as disclosed in Japanese Patent Unexamined Publication No. 57-36752. The cathode structure for a magnetron shown comprises a helical filament 1 for emitting thermal electrons, an upper end shield 2 for holding the upper end of the filament 1, a lower end shield 3 for supporting the lower end of the filament 1 and provided with a hole 3a in the center thereof, a center support 4 disposed within the hole 3a of the lower end shield 3, the center support 4 supporting, at one end, the upper end shield 2 and serving as a lead conductor through which a predetermined amount of power is supplied, a side support 5 supporting, at one end, the lower end shield 3 and also serving as a lead conductor for supplying the predetermined amount of power, and getters 6 fixed on the upper and lower end shields 2 and 3 by welding, respectively. The other end of each of the center and side supports are connected to a power supply not shown.
In the cathode structure for the magnetron thus configured, the filament 1 wound around the center support is ordinarily formed of a thorium-tungsten wire, the getter 6 is formed of metal such as zirconium or titanium, and the upper and lower end shields 2 and 3, the center support 4 and the side support 5 are formed of metal having a high melting point such as molybdenum or tungsten. In this structure, there are junctions between both ends of the filament 1 and the upper and lower end shields 2 and 3, a junction between the upper end shield 2 and the center support 4, and a junction between the lower end shield 3 and the side support 5. A bonding operation is applied to these junctions by using eutectic alloy brazing materials 7 of ruthenium and molybdenum to electrically an mechanically join together respective member.
However, it has been experimentally confirmed that the thus configured conventional cathode structure for the magnetron includes various kinds of problems described below:
(1) Since the getter 6 is formed as an approximately 0.05 mm thick, disk-shaped thin plate having a small aperture in the central portion thereof, it is likely to be deformed and warped owing to cleaning, heat treatment, or handling after press forming work.
(2) When joining together the upper end shield 2 and the getter 6 by spot-welding, a large warping of the getter 6 as previously mentioned tends to cause a welded portion to break and float, and the warping of the getter 6 makes it difficult to carry out welding work.
(3) In operations of the magnetron, heat transfer from the upper end shield 2 to the getter 6 through welded portions differs from that through not-welded floating portions. Thus, this leads to non-uniformity in temperature of the getter 6, with the result that gas absorption effect of the getter 6 becomes non-uniform and a speed at which the getter 6 reacts with gas molecules is lowered. Further, there exists non-uniform inter-diffusion at contact portions between the upper end shield 2 and the getter 6, causing the getter 6 to be deformed remarkably and to be partially stripped off from the upper end shield 2.
(4) The getter material is oiled during press forming and oil remains deposited on the surface of the getter. When cleaning the getter 6 for removal of oil therefrom after pressing work forming, getters 6 stacked one after another tend to adhere closely to each other through oil, with the result that removal of oil by cleaning becomes insufficient. Further, the close adherence of the getters is aggravated during the subsequent heat treatment, thus making it extremely difficult to separate and supply getters 6 for the welding work. In such a case, if an attempt is made to forcedly supply getters 6, they will be greatly deformed.
Because of the drawbacks with the conventional cathode structure as described above, the lifetime of the conventional magnetron is greatly shortened and the fabrication working efficiency thereof is low.